Simultaneous measurement of nucleotide occupancy and mechanical displacement in myosin-V, a processive molecular motor.

Adenosine triphosphate (ATP) turnover drives various processive molecular motors and adenosine diphosphate (ADP) release is a principal transition in this cycle. Biochemical and single molecule mechanical studies have led to a model in which a slow ADP release step contributes to the processivity of myosin-V. To test the relationship between force generation and ADP release, we utilized optical trapping nanometry and single molecule total internal reflection fluorescence imaging for simultaneous and direct observation of both processes in myosin-V. We found that ADP was released 69 +/- 5.3 ms after force generation and displacement of actin, providing direct evidence for slow ADP release. As proposed by several previous studies, this slow ADP release probably ensures processivity by prolonging the strong actomyosin state in the ATP turnover cycle.